This invention relates to improvements in the catalytic cracking of hydrocarbons and, in particular, is directed to a process for the catalytic cracking of hydrocarbon oils to produce higher light olefin yields and increased gasoline octane number. The cracking catalyst used herein is a mixture of a large pore crystalline molecular sieve such as zeolite Y and a zeolite referred to herein as zeolite ZSM-57.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724; 4,073,865; and 4,104,294 describe crystalline silicates of varying alumina and metal content.
The catalytic cracking of hydrocarbon oils utilizing zeolites is a known process, practiced, for example, in fluid-bed catalytic cracking (FCC) units, moving bed or thermofor catalytic cracking (TCC) reactors and fixed bed crackers. Zeolites have been found to be particularly effective for the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified U.S. patents.
It is also known that improved results can be obtained with regard to the catalytic cracking of gas oils if a zeolite having a pore size of less than about 7 Angstrom units, e.g., zeolite A, is included with a crystalline zeolite having a pore size greater than about 8 Angstrom units, e.g., rare earth-treated zeolite X or Y, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than about 7 Angstrom units into a catalyst composite comprising a large pore size crystalline zeolite (pore size greater than about 8 Angstrom units) has indeed been very effective with respect to raising the octane number, it does so at the expense of overall gasoline yield.
Improved results in catalytic cracking with respect to both octane number and overall gasoline yield are disclosed in U.S. Pat. No. 3,758,403. The cracking catalyst comprises a large pore size crystalline zeolite (e.g., pore size greater than about 8 Angstrom units) such as zeolite Y in admixture with a smaller pore zeolite, e.g. ZSM-5, wherein the ratio of smaller pore zeolite to large pore size crystalline zeolite is in the range of 1:10 to 3:1. Effective cracking was achieved when the catalyst was used to obtain the inherent advantages realized in moving bed techniques such as the Thermofor Catalytic Cracking Process (TCC) as well as in fluidized cracking processes (FCC).
The use of zeolites such as ZSM-5 in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of a ZSM-5 zeolite in amounts of about 5-10 wt. %; the latter patent discloses the weight ratio of ZSM-5 zeolite to large pore size crystalline zeolite within the range of 1:10 to 3:1.
The addition of a separate additive or composite catalyst comprising one or more members of a class of zeolites such as ZSM-5 has been found to be extremely efficient as an octane and total gasoline yield improver when used in very small amounts in conjunction with a conventional cracking catalyst. Thus, in U.S. Pat. Nos. 4,309,279; 4,309,280; and 4,368,114, it was found that only 0.1 to 0.5 wt. % of ZSM-5 added to a conventional cracking catalyst under conventional cracking operations could increase octane by about 1-3 RON (Research Octane Number Without Lead).
U.S. Pat. No. 4,740,292 discloses a catalytic cracking process which employs a mixture of a faujasite-type zeolite as base cracking catalyst and zeolite Beta. Use of this catalyst mixture results in improved cracking activity, increased octane of the product gasoline and increased gasoline plus alkylate-precursor yields relative to the base catalyst alone.